Torrance, CA--Scientists from Opto-Knowledge Systems, Inc. (OKSI; Torrance, CA), Pacific Northwest National Laboratory (Richland, WA), and Rutgers University (Piscataway, NJ) have developed a new type of hollow-core optical fiber that can deliver mid-infrared (mid-IR) as well as long-wavelength infrared (LWIR) laser light. The fibers, detailed in "Fiber delivery of mid-IR lasers" from the SPIE Newsroom (August 24, 2011), are important for mid-IR light delivery from quantum-cascade lasers (QCLs) and other mid-IR sources in instruments that can detect trace molecular species in such applications as explosives detection, environmental pollutant monitoring, and in analysis of biomarkers in a patient’s breath.

IMAGE: The basic structure and coating layers--silver (Ag) and silver iodide (AgI)--of the mid-IR and LWIR hollow fibers are shown. (Courtesy SPIE Newsroom)

Although mid-IR light delivery is possible using solid-core chalcogenide glasses, these chalcogenide fibers are extremely brittle if impurities are not controlled. In addition, transmission is attenuated strongly at long wavelengths, with a working limit of about 6 microns for sulfide and 9 microns for selenide glass types of chalcogenide fibers.

The hollow-core fibers were manufactured by pumping a silver solution through glass tubing, creating a smooth silver layer on the inside of the tube. An iodine solution is then pumped through the tube, which forms a dielectric silver iodide (AgI) layer upon reaction with the silver. The glass tubing provides a smooth, robust, and flexible structure on which to deposit the coatings. For most mid-IR applications, AgI is used as the dielectric material, and its thickness can be optimized for specific wavelengths. In general, the process produces two broad classes of fibers: single-mode fibers for long-wave infrared (LWIR) applications operating from 6 to 14 microns, and multimode, low-loss fibers for broadband applications operating between 3 and 14 microns.

In addition to preserving a Gaussian profile on bending, the single-mode fibers exhibited mode filtering; a beam from a QCL with an elliptical shape and fringes was cleaned-up by the hollow fiber, resulting in a circularly symmetric (Gaussian) output.

SOURCE: SPIE Newsroom; http://spie.org/x52092.xml?ArticleID=x52092